INVESTIGATIONS INTO THE FORMATION AND CHARACTERIZATION OF PHOSPHOLIPID MICROEMULSIONS .2. PSEUDO-TERNARY PHASE-DIAGRAMS OF SYSTEMS CONTAINING WATER-LECITHIN-ISOPROPYL MYRISTATE AND ALCOHOL - INFLUENCE OF PURITY OF LECITHIN

The phase behaviour of quaternary systems composed of lecithin/isopropyl myristate/water/ and one of seven short-chain alcohols at various surfactant/cosurfactant mixing ratios (K-m) has been investigated by the construction of phase diagrams. A commercially available soybean lecithin (namely, Epikuron 170, phosphatidylcholine purity 68-72%) was used in the study. Phase diagrams showed the area of existence of a stable isotropic region along the surfactant/oil axis (i.e., reverse microemulsion area; L(2)) in all systems regardless of the K-m. The existence of a second water-rich isotropic region (i.e., normal microemulsion area; L(1)) was, however, seen to be very dependent upon the K-m and occurred in only a few instances. This second isotropic region, L(1), always occurred in conjunction with a liquid crystalline domain, although in some cases, particularly at the lower K-m, a liquid crystalline region was seen to occur without the presence of an L, phase. Comparison of the results with those obtained using higher purity lecithins (greater than 92% phosphatidylcholine content) indicated that while the phase diagrams were qualitatively similar, significant differences occurred at oil levels below 50%. It was found that the existence of an L(1) and LC region and the extent of the L(2) region were dependent upon both the purity of surfactant and the surfactant/cosurfactant mixing ratios (K-m).